The fluctuations of the voltage across the resting membrane of myelinated nerve fibers have been analyzed. They show a 1/f spectrum and a Gaussian amplitude distribution and are related to the net flow of potassium through the membrane. When the average membrane voltage is made more negative by means of an external current, depolarizing deflections can be observed. They cause an asymmetry in the noise amplitude distribution and a marked increase in the subaudio spectral components of the noise. The depolarizing deflections can be attributed to batch-wise inflow of sodium ions. Possible mechanisms of both types of membrane voltage fluctuations are discussed.